Name ______________________________________________ Date _____________ Per ____

 

Demonstration Lab – Osmosis and Diffusion

 

Terms to Know:

§  Solute – material or particles that are dissolved in a liquid (i.e. sugar).

§  Solvent – the liquid that the above material is dissolved in (i.e. water).

§  Solution – the combination of the solute and the solvent (i.e. sugar water).

§  Hypotonic – an area of lesser solute concentration.

§  Hypertonic – an area of higher solute concentration.

§  Isotonic – an area of equal solute concentration.

§  Osmometer – a meter used to measure the process of osmosis.

 

Introduction:

The molecules that make up solids, liquids, and gases are in constant motion.  They move from areas of higher concentration to areas of lower concentration by the process of diffusion.  Molecules must also be able to diffuse into and out of living cells, providing a source of nutrients and allowing for the export of cell products or wastes.

 

A cell is surrounded by a cell membrane made mostly of lipids and proteins.  The membrane is selectively permeable because it allows some materials, but not others to move across the membrane.  Small ions and molecules of oxygen (O2), carbon dioxide (CO2), water (H20) can move across freely.  Other molecules, especially those that do not mix well with lipids (oily or fatty substances) and large macromolecules, must move through special pores or channels in the cell membrane made up of proteins.

 

The movement of water through a selectively permeable membrane is a special type of diffusion called osmosis.  Water moves from an area of lesser solute concentration (hypotonic) to an area of greater solute concentration (hypertonic).   

 
 

                                                   Solute                                                                       Hypotonic Solution


 

                                                                        Solution                                                     

 

  Solvent                                                                     Hypertonic Solution

 Hypotonic          Hypertonic

               

                 Membrane

 

§  Animal cells must be surrounded by an isotonic solution in order to live.  If placed in pure water, the water will move into the cell and the cell will expand until in bursts (cytolysis).  The cell membrane is very elastic (like a balloon).

 

§  Plant cells, however, must be surrounded by a hypotonic solution in order to live.  When placed in water, the water will move into the plant cell, but the cell wall surrounding the cell membrane is not very expandable.  Pressure builds up within the cell from the influx of water.  The pressure or force directed against the cell wall is called turgor pressure.  If you put limp celery or a wilted flower into water, the cells will take up water and become turgid

 

Water molecules, in the process of osmosis never stop moving.  Even when the concentration of solute is equal on both sides (isotonic), the water molecules move in and out of a cell at an equal rate.  The same number of water molecules move in and out, so the system remains in equilibrium.

Part I:  Egg Osmosis

In the lab, we will explore the movement of water into and out of a cell, by using an egg as an osmometer (a meter to measure the process of osmosis).  Remember that an egg is a single cell, a very large single cell.  Using our egg osmometers, we will measure the effects of hypertonic and hypotonic solution on animal cells.

 

Use the information you just read to answer the following questions.

1.      If more water moves into an egg than moves out, you would expect the egg to have a __________________ mass than before it was placed in a solution.  Would the solution surrounding the egg be hypertonic or hypotonic compared to the solution (cytoplasm) inside the egg? ___________________________________

 

2.      If more water were to move out of the egg than in, you would expect the egg to have a _________________ mass than before it was placed in a solution.  Would the solution surrounding the egg be hypertonic or hypotonic compared to the solution (cytoplasm) inside the egg? ___________________________________

 

Purpose:

What will happen to eggs when we put them in different concentrations of sugar solutions? 

§  The sugar that we are using is sucrose.  Its molecules are too large to pass though the egg (cell) membrane. 

 

§  If sugar can't move across the cell membrane, then what molecule moves across the cell membrane to change the concentration inside or outside of the cell?  __________________________

 

3.      Make observations of the eggs.  Other than not having their shell anymore, does it look like it has changed in anyway?  If so, how?

____________________________________________________________________________________

 

4.      Use your observations to fill in the table below.  Determine which solution was hypertonic and which was hypotonic

 

% Sugar Concentration

Apparent Change of Mass (gained or lost)

Hypotonic or Hypertonic (solution surrounding the egg)

Egg 1

0% SUGAR

 

 

Egg 2

50% SUGAR

 

 

 

Part II:  Iodine and Starch

A dialysis tube is similar to a cell membrane in that it allows certain molecules to pass through, but keeps other molecules out.  A starch solution is placed inside the dialysis tubing and then sealed.  The tube is then placed in an iodine solution.  Iodine, a yellow-brown liquid, turns bluish black when mixed with of starch. 

 

Examine the iodine and starch set up.  Answer the questions below.

5.      Observe the beaker of water in the initial set-up the does not have the dialysis tubing.  What color is the water in the FINAL set-up?  ____________________

6.      Observe the dialysis tube in the beaker in the initial set-up.  What color is it at the FINAL set-up?  _________________________________________________________________________________________

7.      Why do you think that the inside of the tube is this color?  _______________________________________ ______________________________________________________________________________________

8.      Why is the jar not a bluish-black color? ______________________________________________________ ______________________________________________________________________________________

9.      What two molecules were small enough to passed through the membrane? ___________________________

Part III:  Osmosis in Beets

Recall the information given in the introduction about hypotonic, hypertonic and isotonic solutions and osmosis of water.  Observe the beet slices placed in the culture dishes in the demonstration.  In one of the dishes the beet was placed in tap water; in the other, the beet was placed in 10% salt water. 

 

Notice the difference in the colors of the water in the two culture dishes.   Describe these differences:

Beet:  Tap Water Observations

Beet: Salt Water Observations

 

 

 

 

 

10.  What occurred with the pigment in the beet in the salt water set-up? ___________________________________ _________________________________________________________________________________________

Part IV:  Osmosis in Carrots

When the vegetables in your salad wither, they do so because of osmosis.  In this part, you will determine how osmosis affects plants.

 

Examine the two beakers labeled “Salt Water” and “Fresh Water.”  The two carrots sticks placed in solution were of the same relative size, and have sat undisturbed for 24 hours.   Examine the carrot sticks for the tightness of the threads and squeeze the carrot stick to determine its texture.

 

DO NOT BREAK THE CARROT STICK!

 

Place a check in the appropriate box below.

Condition of Carrot Stick

Present in salt water?

Present in Fresh water?

§  Thread is loose

 

 

§  Cells gained water

 

 

§  Soft texture

 

 

§  Thread is tight

 

 

§  Firm Texture

 

 

§  Cells lost water

 

 

 

11.  Did water move into or out of the carrot in the salt water? _____________________________

12.  Did water move into or out of the carrot in the tap water? _____________________________

13.  One way to prevent salad and vegetables from wilting is to cover them with plastic wrap.  Why is this?  (Explain in terms of osmosis.)  _____________________________________________________________ ______________________________________________________________________________________________________________________________________________________________________________

14.  Supermarket workers spray fruits and vegetables with water to make them more desirable to consumers.  Why does spraying vegetables with water prevent them from drying out? __________ __________________________________________________________________________________________________________________________________________________________________________________


 

REVIEW QUESTIONS:

 

1.      Why is it important for the cell membrane to be selectively permeable?

 

 

2.      Why do animal cells need to be surrounded by isotonic solutions?

 

 

3.      Why do plant cells need to be surrounded by hypotonic solutions?

 

 

                   

4.      What is turgor pressure?   Why do plant cells not burst when placed in pure water?

 

 

 

 

5.      What would happen to a cell if it were placed in a hypertonic solution?  Why? (drawing the diagram may help)

 

 

 

6.      Does osmosis ever “stop”? Explain your answer.

 

 

                          

7.      Look at the solution concentration below the diagram.    Draw in the appropriate concentration of solute on either side of the semi-permeable membrane in the diagram below.

 

 

      Semi-permeable Membrane

  

 

 

 

 

 

 

 

 

 

 


                                            Solution A (5%)                                   Solution B (20%)

                                           Water (95%)                                        Water (80%)

 

 

8.      Draw an arrow on your diagram to show which way the water (solvent) will move.  Be sure to label the arrow with the word ‘water’. 

 

 

9.      If we assume that the solute was able to pass through the membrane, draw a second arrow on your diagram to show direction the solute would move.  Be sure to label the arrow with the word ‘solute’.